In a traditional cataract refractive surgery, the refractive astigmatic axis of the cornea anterior surface, the cornea, or the eye of a patient is measured or determined prior to surgery. In the following the term astigmatic axis defines the location of the axis or meridian of an astigmatic eye. A mark, (using ethylene blue marker, for example), identifying the astigmatic axis is then typically made on the sclera before or during the surgery to guide a surgeon in correcting the eye's astigmatism. For example, when performing a limbal relaxing incision (LRI) or corneal relaxing incision (CRI), the mark can guide the surgeon in determining where to make the incision. If a toric intra-ocular lens (IOL) is implanted, the mark can guide the surgeon in rotating the toric IOL to a desired orientation.
Traditional hand based astigmatic axis marking using a surgical marker pen generally is not accurate and/or precise as the thickness of the pen mark along with the fact that the mark “bleeds”/wicks out over a wider area causes additional meridian error covering an angular range of several to many degrees. In addition, the astigmatic axis measurement which is generally based on keratometry/keratoscopy or corneal topography does not take into account the contributions to astigmatism from the posterior corneal surface and potentially the contribution from the crystalline lens. Furthermore, a whole eye astigmatic axis measurement can have contribution from an astigmatic crystalline lens. All these can lead to unaccounted and uncorrected astigmatism error in a post-operative eye.
In light of the above, there is a need in the art for a better approach to more accurately determine a target axis of astigmatism correction or neutralization in a cataract refractive surgery such that any unaccounted astigmatism can be optimally corrected.